Packers are used in many applications downhole. In one application, packers are run with a liner which is to be hung in the wellbore. In these applications, a running tool is used with a tubing string to position the liner and set the hanger. After the liner hanger is set, it is common practice to release the running tool and raise the work string several feet. The work string is raised prior to cementing the liner to check for weight loss as an indication that the running tool has released from the liner. Thereafter, it is desirable to lower the work string and apply a downward force through the running tool and into the liner during cementing so as to counteract the upward forces due to hydraulics when the cement is pumped through the liner. When a compression-set liner top packer and/or another mechanical or hydraulic device in the string is used, a packer-setting device which consists of a dog sub is usually positioned just above the running tool. The dogs are compressed to fit inside an extension of the weight-set packer during run-in.
In the past due to deviation in wellbores, it was uncertain as to where the dogs would wind up when pulling up at the surface to ensure a release of the running tool from the liner. The problem in prior designs was that if the dogs were pulled out of the extension of the weight-set packer when testing for release of the running tool from the liner, a subsequent downward force would prematurely set the packer prior to the onset of the cementing job or prematurely locate the running string opposite another device which is mechanically or hydraulically operated. One solution that had been implemented was to put an unduly long extension tube on the weight-set packer to ensure that no matter how high (within reason) the dog sub was lifted that the dogs would remain inside the tube so that the string could be set down to resist the hydraulic forces occurring during the cementing operation. However, lengthening the extension tube provided a restriction against circulation of mud being displaced by the advancing cement. This would prolong the time required to complete the cementing and thus increase costs for the well owner. It could also adversely affect the quality of the cementing job.
In the past, dog subs such as disclosed in U.S. Pat. No. 5,813,458 ("the '458 patent") could be employed to facilitate the operations of cementing, followed by setting the weight-set packer. Furthermore, as disclosed in the '458 patent, a shear screw in the assembly could be used as a signal to surface personnel when extracting the dog sub to the surface that a sufficient force had been applied to the weight-set packer. However, the dogs in that patent were free to engage the extension tube if they inadvertently got pulled out of the extension tube during the procedure to determine that the running tool had released from the liner.
Accordingly, one of the objectives of the present invention is to provide a dog sub which can literally come out of a reasonably sized extension tube without release of the dogs to facilitate the step of determining that the running tool had released from the liner. Another objective is to then allow the dogs to be subsequently released when inside the extension so that thereafter, when removed from the extension, they can be used to set the weight-set packer and to provide the necessary signal to surface personnel that sufficient weight had been used to set the weight-set packer.
Another objective of the present invention is to facilitate the sequence of operations which involve setting the liner hanger, picking up the string as much as necessary to ensure disconnection of the running tool from the liner, setting back down into the liner and holding that position during cementing, picking up to allow the dogs to engage the top of the extension tube, setting down weight to set the packer, and to provide a signal to surface personnel that sufficient force has been applied to the weight-set packer. Accordingly, the objective of the present invention is to eliminate uncertainties of the prior art devices which could have catastrophic consequences if, during the pickup of the string to release the running tool from the liner, the dogs emerged form the extension tube on the packer. Should that occur, the running string would have to be pulled out of the hole and the dogs removed, followed by another trip back in the hole with the running string to facilitate the cementing job. Thereafter, the running string would have to be tripped out of the hole and some object located on the bottom end of the running string so that it would ultimately engage the extension tube on the weight-set packer to subsequently set the packer after the cementing job. The problem would be that the time delay involved in these extra trips would cause the formation to come back in around the liner, thus making circulation difficult for placement of the cement around the liner. On top of that, there are operational uncertainties of whether the weight-set packer could even be actuated at the conclusion of the cementing job with makeshift tools. Accordingly, one of the objectives of the present invention is to allow extraction of the dog sub from the extension tube without adverse consequences for the purpose of determining release of the running string from the liner. Those and other advantages of the method and apparatus of the present invention will become more apparent to those skilled in the art from a review of the detailed description of the preferred embodiment which appears below.